Got Bt Resistance?

Modified toxins might be key to fighting resistant bugs

What do Bt-resistant pink bollworms found in fields in India have to do with U.S. agriculture? A lot.

In 2008, the discovery of Bt-resistant pink bollworm in India wasn’t a big surprise. Insects are expected to evolve resistance to Bt technology or other control tactics. However, the case in India was special because it allowed researchers to explore how resistant insects differ from susceptible insects at a basic molecular level: their genes. What researchers found holds warning for all U.S. agriculture and is vital to delaying further resistance in Bt crops.

Jeffrey Fabrick, Research Entomologist, USDA–Agricultural Research Service, Maricopa, Ariz., and Bruce Tabashnik, head of the Department of Entomology, University of Arizona, have worked for many years to understand resistance to Bt in insects. In fact, their teams have selected Bt-resistant pink bollworm in the laboratory, generating invaluable information.

"Susceptible insects have a protein in their gut that binds Bt toxin," Tabashnik says. "This binding is an essential step in how Bt toxins kill insects. In the resistant lab strains, we discovered the gene for cadherin was disrupted, yielding altered cadherin protein that does not bind Bt toxin."

Think of Bt toxin as a key and cadherin as a lock. In resistant insects, the cadherin "lock" is changed so the Bt toxin "key" no longer fits. The University of Arizona and USDA teams have been screening insects in Arizona for more than 15 years. After analyzing DNA of thousands of samples, they found some with variations in cadherin, but only four different changes were identified. However, after sequencing the DNA from just eight specimen from India cotton fields, they found 19 variants—a stunning difference.

"When we learned pink bollworm had evolved resistance in India, our first question was, ‘Would the genetic changes in India match the ones we’d discovered in the lab in Arizona?’" Tabashnik says. The answer is mixed. There were mutations in the same gene, but the changes in India were entirely different from those in Arizona. Not a single Indian sample showed the same changes seen before, and the changes to cadherin in India pink bollworm were wildly diverse.

Research gold. This news is mixed for agriculture. Finding the same gene altered in India and Arizona provides hope that patterns of resistance can be identified, tracked and countered. But the specifics did not match those seen in Arizona and couldn’t be predicted.

"What we found is remarkable," Fabrick says. "Although resistance can happen anywhere, these resistant insects in India are not a direct threat to U.S. fields. But these insects are absolutely amazing in how they can adapt to different controls. It’s not unexpected to see resistance. In fact, most people are surprised at how long we’ve been able to maintain the efficacy of Bt crops. But we want to extend the life of this technology, and that means being sure these crops are used in the most appropriate fashion."

Fabrick and Tabashnik might have found one way to counter insect resistance with new genetically modified Bt toxins in Bt cotton. The lab results show the modified toxins kill the resistant strains from Arizona.

"It’s like spy versus spy," Tabashnik says. "We implement a new control measure, the insects adapt and we counter with something that overcomes their adaptation."

Because the resistant insects in Arizona and India have similar changes, the researchers hope the modified toxins will fight the Indian insects. But the modified toxins haven’t been tested in the field or on the insects from India.

Scientific literature reports five cases of insects with field-evolved resistance globally, Fabrick explains: fall armyworm in Puerto Rico, corn stem borer in South Africa, corn rootworm in the U.S., corn earworm in the U.S. and pink bollworm in India.

"Unfortunately, we don’t have much information in these cases, except for the India pink bollworm, regarding what is going on at the genetic level in the field that allows these insects to survive on Bt crops," he adds.

Lack of refuge use in India was likely a crucial factor enabling resistance to flourish. "This is important to U.S. corn farmers because the results imply that refuges of non-Bt crops are helpful in delaying insect resistance to Bt crops," Tabashnik says. His main message for agriculture is not to rely on any single tactic: The best way of slowing resistance to an insecticide or a Bt crop is combining a variety of tactics through integrated pest management.